¡Qué milagro!

What can you say when running into someone you haven't seen in a while? Here's a good standby option: ¡Qué milagro! = Long time, no see.

It doesn't matter whether your chance meeting is much of a miracle or not. You can say it with genuine joy at seeing an old friend — or with reproach, if it's someone who hasn't been returning your messages.

If you comment on how long it's been, note that the past perfect rather than the present perfect is normal. No te había visto en mucho tiempo, ¿verdad? We haven't see each other in ages. Spanish is the more logical language here, since the period of not seeing ended when you ran into each other.

Qué milagro also fits when someone calls or messages you out of the blue.

— Habla María. This is Maria (on the phone).
— Hola, María. ¡Qué milagro! How nice to hear from you.

Toparse

By the way, running into someone (an unplanned meeting) is encontrartse/toparse con alguien: Me topé con tu hermana en la calle. I ran into your sister in the street. If the meeting was planned, use something like verse instead: Nos vimos en un café. We got together (for a planned date) in a coffeeshop.

Caerte

When you're talking about dropping in on someone — going to see someone where they are or will already be — use caerle.

• Ahí te caigo. I'll drop by. I'll see you there (where you are).
• ¿Te puedo caer hoy? Can I come over today?

Speak of the Devil

What can you say if you were just talking about someone and they suddenly show up? ¡Hablando del rey de Roma! = Speak of the devil. If the person in question is female, remember to adjust accordingly: ¡Hablando de la reina de Roma!

This expression can be said either directly to the newcomer or discreetly to your conversation partner. It can be humorously extended to rhyme, for example, ... y el wey que se asoma.